1. Field of the Invention
The present invention relates to a method and a device for supplying electric current to ceramic heaters, for example used as glow plugs to facilitate a starting of a diesel engine.
2. Prior Art
Conventionally, high voltage V.sub.1 is applied in the initial current supplying period to abruptly heat the ceramic glow plug for a diesel engine, for example to about 900.degree. C. in about three seconds after every starting of the engine as shown in FIG. 12. When the temperature of the glow plug reaches about 900.degree. C. (at which temperature the sintered body of the glow plug is not cracked and the glow plug can perform ignition), low voltage V.sub.2 is applied to maintain the stable saturation temperature (about 1,150.degree. C.). Then current supply is stopped. In this way, one cycle is completed to facilitate the starting of the diesel engine.
In this one cycle or in the repetition of such a cycle for an extended period, DC voltage is generally applied, with its polarity being fixed, from a battery to the glow plug. Accordingly, when such voltage is repeatedly applied in the method of supplying electric current of this type, substance movement occurs in the ceramic sintered body highly heated at 1,150.degree. C. or more, resulting in generation of cracks in the ceramic sintered body. Therefore, it is difficult to use the glow plug at high temperatures.
Generally, it is well known that the cracks in the ceramic sintered body of a ceramic heater causes the breakage of the ceramic sintered body and the ceramic heater cannot function properly. This is a serious problem in use of the ceramic heater for a diesel engine's glow plug which must be highly reliable and durable. Accordingly, a ceramic heater which does not cause cracks in its ceramic sintered body even after its repetitive use for an extended period is desired to be developed.